'We found that girls who are African-American matured before whites, and that's been shown in several studies,' Biro tells WebMD. 'White girls are maturing earlier than they had before, compared to 20 years earlier.'

In his study of 1,239 girls, 10% of whites, 23% of African-Americans, and 15% of Hispanic girls had breast development indicating onset of puberty by age 7, Biro found.